Fossil fuel is a general term for buried combustible geologic deposits of organic materials formed from decayed plants and animals that have been converted to crude oil, coal, natural gas, or heavy oils by exposure to heat and pressure in the earth's crust over hundreds of millions of years. Fossil fuels are a finite and non-renewable resource.
Increased demand for energy by the global economy has also placed increasing pressure on the cost of hydrocarbons. Aside from energy, many industries, including plastics and chemical manufacturers, rely heavily on the availability of hydrocarbons as a feedstock for their manufacturing processes.
Plant biomass (e.g., wood, wood chips, cuttings, grasses, crop residues, etc.) is a renewable energy source (see, e.g., U.S. Pat. App. Pub. Nos. 20110123407 and 20020132972, incorporated herein by reference) but has drawbacks as a feedstock for fuel and chemical manufacturing. One such drawback is the natural moisture level of the plant biomass. For plant biomass to be useful as a fuel, the moisture level needs to be reduced, requiring energy input, usually in the form of heat for drying. Another drawback of plant biomass is that plant biomass typically contains upwards of 70% in air and void space, which makes it inefficient to collect, store, ship and use plant biomass for fuel and other applications.
One example of how plant biomass has been used as a renewable energy source is the artificial (or synthetic) firelog industry, which produces fuel logs and pellets for fireplaces and stoves. Such products have been made from wax and sawdust and/or other cellulosic materials, such as rice hulls or shredded paper, with a combustible material to facilitate ignition (see U.S. Pat. Nos. 3,297,419; 4,104,034; and 6,136,054). Because the wax is often derived from crude mineral oil, a non-renewable and ever more costly source of material, fireplace logs containing cellulosic materials, such as liquid combustible by-products derived from treatment of vegetable and animal materials, have been described but require special processing (see U.S. Pat. No. 4,326,854). Efforts to avoid the costs and problems associated with the use of sawdust, paper products, and wax in such products have included attempts to use materials such as spent coffee grounds to replace all or part of these components (see U.S. Pat. No. 5,910,454). The difficulties of acquiring such products in the amounts needed for large scale production, however, have limited the adoption of such materials and methods.
Another example of a renewable energy source is the generation of microbial oil (“bio-oil”) from fast pyrolysis of Chlorella protothecoides (see Miao and Wu, 2004, J. Biotechnology 110: 85-93, incorporated herein by reference) in which these microbes were cultivated and then subjected to fast pyrolysis to yield microbial oil. This process has drawbacks, however, in that it was performed only at laboratory scale, and microbial lipid has higher value applications, including isolation and direct conversion to biodiesel or processing to renewable diesel or jet fuel (see PCT Pat. Pub. No. 2008/151149, incorporated herein by reference).